Observation of two low-temperature double carbonate phases between K and Cd

We have discovered two new phases from the reaction system of CdO(s) and K2CO3(s) under CO2(g, 1 ​atm) at temperatures between 230 and 300 ​°C. Both phases have narrow ranges of temperature of stability: phase 1 is observed at 250–430 ​°C and phase 2 ​at 230–270 ​°C during heating in in situ X-ray diffraction measurements. At higher temperatures than 430 ​°C, the reaction system is composed of CdO and K2CO3 only, indicating the decomposition of the two phases. Probably because of the slow diffusion of solid materials at the low temperatures of stability, both phases are formed as minorities with the majority CdO even after 50 ​h of reactions at their optimal temperatures of stability. 2 is determined to have the high temperature structure of nyerereite of the composition Na1.67K0.33Ca(CO3)2 in the space group P63mc. Rietveld refinements on the X-ray diffraction pattern have produced satisfactory structural model for 2, based on which the composition of 2 is determined to be K2Cd(CO3)2. On the contrary, attempts to identify the structure type of 1 have failed, leaving its composition also undetermined.